A DESCRIPTION OF THE PROBLEM :
## Problem Description

In `URLClassPath.getLoader(int)`, when processing JAR files with `META-INF/INDEX.LIST`, dependent JARs are pre-populated into the `lmap` with `null` values during `JarLoader.ensureOpen()`. Later, when `getLoader(int)` encounters these URLs, it skips creating actual Loaders because `lmap.containsKey()` returns `true`, but the value is `null`. This results in resources from these JARs being permanently unavailable.

## Root Cause

1. `JarLoader.ensureOpen()` pre-populates `lmap` with `null` for dependent JARs from INDEX.LIST
2. `URLClassPath.getLoader(int)` only checks `lmap.containsKey()` without verifying the actual value
3. JARs with `null` values in `lmap` are skipped and never added to the `loaders` list

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
## Reproduction Steps

1. **Development Environment**:
   - JDK-17.0.16+8
   - Apache Maven 3.9.1

2. **Create Maven Project**:
   - Create a Maven project named "ResourcesProject"
   - Ensure the dependency `org.springframework:spring-context:6.1.13` is included
   - Configure `maven-jar-plugin` in `pom.xml` to include `<index>true</index>`

3. **Additional Configuration**:
   - Configure `maven-jar-plugin` with `<classpathPrefix>lib</classpathPrefix>`

4. **Test Code**:
```java
public class ResourcesLoadTest {
    public static void main(String[] args) {
        ClassLoader cl = ResourcesLoadTest.class.getClassLoader();
        String resourceName = "META-INF/spring.schemas";
        Enumeration<URL> urls = null;
        try {
            urls = cl.getResources(resourceName);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        int count = 0;
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            System.out.println(url);
            count++;
        }
        System.out.println("Total: " + count);
        assert count >= 3;
    }
}
```

5. **Deployment and Execution**:
   - Compile and package the Maven project
   - Deploy to a Linux system
   - Ensure `ResourcesProject.jar` is the first item in the classpath
   - Execute with command: `java -ea -cp "ResourcesProject.jar:lib/*" ResourcesLoadTest`

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
   - **Expected Result**: `AssertionError` will be thrown because the actual count of loaded resources is less than 3
ACTUAL -
jar:file:/home/liuyan1001/ResourcesProject/lib/spring-context-6.1.13.jar!/META-INF/spring.schemas
Total: 1
Exception in thread "main" java.lang.AssertionError
        at com.ubitrans.ResourcesProject.ResourcesLoadTest.main(ResourcesLoadTest.java:24)


---------- BEGIN SOURCE ----------
public class ResourcesLoadTest {
    public static void main(String[] args) {
        ClassLoader cl = ResourcesLoadTest.class.getClassLoader();
        String resourceName = "META-INF/spring.schemas";
        Enumeration<URL> urls = null;
        try {
            urls = cl.getResources(resourceName);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        int count = 0;
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            System.out.println(url);
            count++;
        }
        System.out.println("Total: " + count);
        assert count >= 3;
    }
}
---------- END SOURCE ----------